Mid supply reference voltage generators are typically made up of a voltage divider and an op-amp. The voltage divider includes impedance elements, such as resistors, and generates a voltage level proportional to the ratio of these impedance elements. The op-amp is configured in a unity gain feed back arrangement connected to the voltage divider.
In these circuits, if the supply current needs to be low, the voltage divider is constructed from large resistors or long-channel MOSFET elements, both of which take up considerable silicon area on an integrated circuit (IC). Additionally, the high output resistance of the voltage divider results in significant thermal noise. The voltage divider is also susceptible to noise coupled from adjacent on-chip circuitry.
These problems can be partially eliminated through the use of a bypass capacitor. However the use of a bypass capacitor is limited by the silicon area available and the stabilization time requirements of the application in which the mid supply voltage generator is employed. Additionally, use of a capacitor increases the time period necessary for the voltage generator to stabilize. This occurs because the bypass capacitor, with the output resistance of the voltage divider, creates a long time constant which significantly limits the applications that can employ the voltage divider For example, in battery powered devices such as cellular radiotelephone products, palm top devices and laptop computers, settling time upon "power-up", or exiting power save mode, is an important characteristic of a supply voltage generator. In these applications, a large time constant is not desirable.
Use of an op-amp also has several disadvantages. Op-amps have an offset voltage which, for most designs, varies with temperature. Op-amps also draw a significant supply current. Op-amps employ a biasing circuit which also draws a significant amount of current. These high current drains are problematic in battery powered devices, wherein it is desirable to have the lowest possible current drain to obtain long battery life.
In a complex mixed signal IC, several different mid supply references may be required, entailing a variety of load impedances and currents. Usually there is no single op-amp that will satisfy all of the requirements of the op-amp in such an application economically. As a result, custom op-amps, having desired frequency compensation and bias circuitry will have to be designed for each application's requirements.
Thus it is time consuming to develop, and expensive to provide, a suitable mid supply voltage generator, especially in battery powered devices. Accordingly there is a need for a mid supply voltage generator that does not have the disadvantages of existing circuits.